1. Field of the Invention
This invention relates to improvements in light collection systems providing light beams for projectors, such as light valve projectors.
2. Description of the Prior Art
Typical prior art color projection systems of the light valve type, as shown in U.S. Pat. No. 3,290,436, W. E. Good et al., and U.S. Pat. No. 3,325,592, W. E. Good et al., both assigned to the assignee of the present invention, include a light collection system having an arc lamp located at the adjacent focus of a simple ellipsoidal reflector. A light beam is reflected from the ellipsoidal reflector through a pair of spaced lens plates having corresponding pluralities of rectangular lenticules stacked into horizontal rows and vertical columns. The second lens plate carries the input light mask of a schlieren optical system, which also includes a light modulating medium and a light output mask, for directing an image onto a viewing screen. With this arrangement, efficient utilization is made of light from the arc lamp, and uniform distribution of light is produced on the light modulating medium.
Present color light valve projectors of this type produce about 250 to 300 screen lumens white light output. There is, however, a need for higher lumen output for many applications. While the use of a more powerful lamp will obviously provide greater light output, lamps rated at 1,000 watts have been found to be a practical limit. Lamps with higher ratings draw too much current for a convenient 20 amp. circuit. The reflector is desirably a "cold" mirror to minimize infrared reflection. If the power of the lamp is too high, the reflector will become overheated and the reflecting surface may be damaged. Lamps of higher rating require larger envelopes which begin to interfere with the light collection efficiency of the reflector unless a reflector of greater size is employed. Also, as the rating of the lamp is increased, the size of the arc and its image increase. If the image becomes too large, it will no longer fit through the slots of the schlieren system and some of the light will be wasted by illuminating the bars between the slots. It is therefore necessary to devise a system which more efficiently utilizes the light output of the lamp.
To this end, attempts to improve light collection from simple, single reflector collection systems have not been fruitful. An increase in the collected flux is possible when the collection angles are symmetrical about the 90.degree. point of the arc lamp emission; a maximum collection angle of about .+-.31.degree. about the 90.degree. point may be attained. Although this is slightly better than the present single reflector design, it is not a meaningful improvement because the flux density variation is worse in that a hot spot is created at the center of the image.
The use of compound reflectors has also been suggested in the past for improving light collection efficiency in apparatus for projecting pictures from transparencies. In U.S. Pat. No 3,078,760, P. J. Brownscombe, improved light collection efficiency is achieved with the use of a reflecting hemisphere concentric with the axis of the arc lamp for collecting the lower half of the lamp emitting radiation and reflecting it back through the arc of the lamp to a main ellipsoidal reflector and auxilliary ellipsoidal reflectors. In order to maintain uniform light distribution in the rectangular light field needed in the projector, Brownscombe found it necessary to truncate opposite sides of the main ellipsoidal reflector, place the auxilliary ellipsoidal reflectors in the truncated areas outside the main reflector, and provide additional semi-elliptical flat mirrors in a chevron-like array below the lamp to receive the image reflected from the auxilliary reflectors. Needless to say, this arrangement is unduly complex.